The abnormal regulation of insulin signalling in insulin-resistant subjects is associated with inadequate suppression of soluble protein tyrosine phosphatase (PTPase) activity in skeletal muscle. Multiple PTPases are likely to be involved in the signalling cascade. SH2-Domain PTPases are a class of PTPases selected as candidates for regulating tyrosine phosphorylation following insulin stimulation since it has been reported that such PTPases associate with several receptors and the post- receptor target, IRS1. SH2-Domain PTPase expression was detected in FAO cells, a rat insulin-sensitive hepatoma cell line, and in C2C12 cells, a mouse muscle cell line, using RT-PCR and degenerate primers. Sequence analysis of the amplified products revealed a high degree of identity with the hematopoietic SH2-Domain PTPase, PTP1C. We observed evidence of alternative splicing of the PTP1C pre-mRNA in human rhabdomyosarcoma cells and in EBV-transformed lymphocytes. This putative alternatively spliced form of PTP1c will be sequenced and characterized. A related PTPase PTP1D, (SH-PTP2, SYP) has been recently found to be expressed in adult liver and skeletal muscle. PTP1D pre-mRNA is alternatively spliced in a region corresponding to the catalytic domain of the PTPase. Relative amounts of the two PTP1D isotypes will be compared in insulin resistant and sensitive subjects. Antibodies specific for this PTPase are commercially available and will be used to compare protein levels and PTPase activity. We plan to take advantage of a tumor cell line, LB T- cell lymphoma (D. Naor), to explore the role of this family of PTPases in insulin mediated signal transduction since these cells have the unique property that they require insulin for growth and they are devoid of IGF2 receptors. These features offer specificity in signalling and reduce the complexity of the signalling pathways to be studied.